Implant telemetry with dynamic tuning

ABSTRACT

Systems and methods for maximizing the resonance frequency match between a reader and a controlled device interacting over a narrowband inductive link involve, in various embodiments, features of the controlled device, the reader, or both.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 62/033,557, filed on Aug. 5, 2014, the entire disclosureof which is hereby incorporated by reference.

BACKGROUND

Wireless charging and patient monitoring are becoming increasinglyimportant in medical applications. By incorporating wireless chargingtechnology, medical implants benefit from greater site flexibility, asmaller total footprint and reduced battery size. However, many hurdlesstill exist for implant devices utilizing inductively based telemetry,including antenna size constraints, the limited number of antennas thatmay be included in an implant, and the expected attenuation caused bytissue that further weakens the inductive coupling link. All of thesefactors must be taken into account to achieve reliable patientmonitoring, consistent actuation of therapy (e.g., drug delivery,electrical stimulation, etc.) and inductive power transfer for chargingthe implant battery.

Telemetry applications include pacemakers, medicine delivery pumps,stimulation devices, and artificial hearts. Implantable drug deliverysystems, for example, which may have a refillable drug reservoir,cannula and check valve, etc., allow for controlled delivery ofpharmaceutical solutions to a specified target. This approach canminimize the surgical incision needed for implantation and avoids futureor repeated invasive surgery or procedures. Refillable ocular drugpumps, for example, usually hold less than 100 μL, are much smaller andmore difficult to access post-implantation than other implantable pumps,such as those used for intrathecal injections or insulin therapy.

Thus, an implantable drug-delivery pump may incorporate telemetry tofacilitate communication with an external monitoring device and wirelesscharging of the battery powering the implanted device via inductivecoupling. The operating parameters of the implantable pump may benon-invasively adjusted and diagnostic data may be read out from thepump to the external monitoring device through wireless signals. Duringa scheduled visit, a physician may place the monitoring device near theimplantable pump and send wireless signals to the implantable pump. Theimplant, in turn, adjusts the parameters in the pump and transmits aresponse command to the monitoring device. Typically, a medicaltelemetry device comprises a coil antenna that transmits and receivessignals using electromagnetic waves. However, other antennaconfigurations known in the field may be utilized as well. A number ofparameters characterizing the efficiency of the coil antenna, e.g., theresonant frequency, gain, quality factor (Q factor), and the thermaleffect (Joule effect or heat) are considered when selecting or designingthe coil antenna.

The wireless power receiver system may comprise additional electroniccomponents such as a battery, a magnetic core, and circuitry includingdata storage and a transceiver for data. Some or all of the circuitry isusually hermetically sealed within a device case, but the telemetry coilmay be placed externally to mitigate any interference caused by certaincase materials. Achieving sufficient power transfer across tissue to animplanted device can pose a major challenge, particularly for smalldevices.

FIGS. 1 and 2 highlight the difference between a small implanted systemwith telemetry and a traditional system such as a radiofrequency ID(RFID) or similar broadband system, in which a tag or other inductivelyresponsive device is powered and interrogated wirelessly by a reader. Asshown in FIG. 1, in a broadband system, the bandwidth of the receiver(i.e., the tag) is sufficiently large for the response to be almost flatacross the transmitter bandwidth, even accounting for tolerances; thatis, while the degree of inductive coupling is not especially high, it isconsistently well above zero across a large frequency band.Consequently, even if the peak transmission frequency varies, it willstill transfer power and/or data to the receiver. Comparatively, in anarrowband inductive link such as those employed by small implantabledevices, the receiver bandwidth is narrow. Hence, as shown in FIG. 2, itmay fail to coincide (or coincide sufficiently) with the transmitterbandwidth, accounting for tolerances, to facilitate adequate power anddata transfer. At the same time, when the transmission and frequencybands do coincide, the system achieves much higher normalized gain.

The resonance frequency (in Hertz) of an inductive link is controlled bya capacitor of capacitance C in parallel with the receiving coil ofinductance L, and its value is given by the following equation:

$f_{0} = \frac{1}{2\;\pi\sqrt{LC}}$

A narrowband system is very sensitive to proper tuning (i.e., thedisparity in f₀ between receiver and transmitter) and susceptible to anyshift of resonance frequency or transmitter frequency drift. The resultof a frequency mismatch in broadband and narrowband systems isillustrated in FIG. 3. In a broadband system, the degradation incoupling efficiency remains low across a relatively wide range offrequencies, whereas in a narrowband system, the coupling efficiencydrops drastically with even a modest mismatch in resonance frequencies.The faster the drop-off, the more limited will be the practicability ofimplementing a narrowband system given realistic manufacturingtolerances and inevitable drifts during operation, as illustrated inFIG. 4, which shows the relative effects of detuning or oscillator driftin broadband and narrowband links. As a practical matter, the lowerlimit of usability is reached when the efficiency falls below 70%, andfor the system represented in FIG. 4, this is a frequency mismatch ofmerely ±1 kHz.

Accordingly, traditional RFID design principles do not readily apply tovery small devices utilizing narrowband links. An RFID transmitter isadjusted so that its frequency matches the resonance frequency to obtainmaximum power transfer. Oscillator frequency drift or detuning effect istypically dealt with by minimizing its amplitude by design (choice ofcomponents, tight tolerance of components) and building the necessarymargin into the link budget to cope with the resulting degradation. Fora well-designed system, less than 50% degradation can be achieved. Inorder to maximize the power received for a given receiver coil orminimize the size of the receiver coil for a given power target, thebandwidth of both the transmitter and receiver are narrowed to what isrequired to still maintain data communication a few kHz in typicalimplementations. This ensures the maximum possible combined Q factor andtherefore the maximum power transfer. Because the bandwidth of both thetransmitter and receiver may only be a few kHz, their resonancefrequencies may need to be accurately matched to avoid significantcoupling degradation.

SUMMARY

In various embodiments, the present invention relates to strategies formaximizing the resonance frequency match between a reader and acontrolled device interacting over a narrowband inductive link. Thesestrategies may involve features of the controlled device, the reader, orboth.

In an aspect, embodiments of the invention feature an implantable devicethat includes or consists essentially of an electronic actuator, acannula for conducting fluid from the electronic actuator to an implantsite, a battery for powering the electronic actuator, control circuitryfor controlling operation of the electronic actuator, and a telemetrysystem for (i) wirelessly receiving commands for controlling operationof the electronic actuator over a narrowband inductive link and (ii)wirelessly receiving a power signal for charging the battery over thenarrowband inductive link. The telemetry system is operatively coupledto the battery and to the control circuitry. The telemetry systemincludes or consists essentially of a memory for storing a resonancefrequency of the system and telemetry control circuitry for wirelesslyreporting the stored resonance frequency in response to an externalinterrogation signal.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The telemetry control circuitry may befurther configured to detect a synchronization signal and, in responsethereto, to detect (i) a succession of transmitted frequencies and (ii)associated amplitudes thereof, and to store data indicative of thedetected frequencies in a log in the memory. The telemetry controlcircuitry may be further configured to receive the resonance frequencyof the system over the narrowband link following transmission of thestored log, and to store the received resonance frequency in the memory.The telemetry control circuitry may be further configured to detect asynchronization signal and, in response thereto, to (i) detect asuccession of transmitted frequencies, (ii) determine an associatedamplitude of each detected frequency, (iii) determine a largest one ofthe amplitudes, and (iv) store the frequency associated with the largestamplitude in the memory as the system resonance frequency. Theelectronic actuator may include or consist essentially of a pump, anelectrical stimulator, and/or one or more sensors.

In another aspect, embodiments of the invention feature a reader forwirelessly communicating with an implanted device over a narrowbandinductive link. The reader includes or consists essentially of aresonator circuit and control circuitry for operating the resonatorcircuit to (i) communicate with the implanted device over a narrowbandlink, (ii) determine a resonance frequency of the implanted device, and(iii) charge the implanted device over the narrowband link at thedetermined resonance frequency.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The control circuitry may beconfigured to determine the resonance frequency of the implanted deviceby wireless interaction therewith. The control circuitry may beconfigured to store the determined resonance frequency in a database(e.g., within a memory of the reader) in association with an identifierof the implanted device. The control circuitry may be configured todetermine the resonance frequency of the implanted device by (i)transmitting a wireless signal whose frequency varies over time and (ii)detecting, during the transmission, a peak amplitude of the wirelesssignal whose frequency corresponds to the resonance frequency. Thecontrol circuitry may be configured to determine the resonance frequencyof the implanted device by (i) transmitting a wireless signal whosefrequency varies over time, (ii) receiving, from the implanted device, alog of entries each having an amplitude corresponding to a frequencydetected during the transmission, and (iii) determining which of theentries corresponds to the largest amplitude. The log may betime-indexed. The resonance frequency may be determined by matching thelog entry corresponding to the largest amplitude with a correspondingfrequency of the wireless signal. The reader may include phase-lockedloop circuitry for maintaining transmission at the resonance frequency.The phase-locked loop circuitry may include or consist essentially of anamplifier, a tuning circuit, an amplitude detector, and a filter. Thetuning circuit may vary a capacitance and/or an inductance of theresonator circuit. The resonator circuit may include or consistessentially of an oscillator, a capacitance, and an inductance.

In yet another aspect, embodiments of the invention feature a method ofintercommunication over a narrowband inductive link between an externalreader and an implanted device. A resonance frequency of the implanteddevice is determined by the reader over the inductive link. Theimplanted device is communicated with and charged over the narrowbandlink at the determined resonance frequency.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The determining step may include orconsist essentially of transmitting, by the reader, a wireless signalwhose frequency varies over time and detecting, during the transmission,a peak amplitude of the wireless signal whose frequency corresponds tothe resonance frequency. The determining step may include or consistessentially of (i) transmitting, by the reader, a wireless signal whosefrequency varies over time, (ii) recording, by the implanted device, alog of entries each having an amplitude corresponding to a frequencydetected during the transmission, (iii) receiving, by the reader, atleast a portion of the log (e.g., entireties or portions of all or someof the log entries) from the implanted device over the narrowband link,and (iv) determining, by the reader, which of the entries corresponds tothe largest amplitude. The log may be time-indexed. Determining theresonance frequency of the implanted device may include or consistessentially of matching the entry corresponding to the largest amplitudewith the corresponding frequency of the wireless signal. Transmission(i.e., between the reader and the implanted device) may be maintained atthe resonance frequency using a phase-locked loop. The determinedresonance frequency may be stored, by the reader, in a database inassociation with an identifier of the implanted device.

The term “substantially” or “approximately” means ±10% (e.g., by weightor by volume), and in some embodiments, ±5%. The term “consistsessentially of” means excluding other materials that contribute tofunction, unless otherwise defined herein. Nonetheless, such othermaterials may be present, collectively or individually, in traceamounts. Reference throughout this specification to “one example,” “anexample,” “one embodiment,” or “an embodiment” means that a particularfeature, structure, or characteristic described in connection with theexample is included in at least one example of the present technology.Thus, the occurrences of the phrases “in one example,” “in an example,”“one embodiment,” or “an embodiment” in various places throughout thisspecification are not necessarily all referring to the same example.Furthermore, the particular features, structures, routines, steps, orcharacteristics may be combined in any suitable manner in one or moreexamples of the technology. The headings provided herein are forconvenience only and are not intended to limit or interpret the scope ormeaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be more readily understood from the followingdetailed description of the invention, in particular, when taken inconjunction with the drawings, in which:

FIGS. 1-4 graphically illustrate the effects of resonance frequencymismatch in broadband and narrowband systems.

FIG. 5 is a block diagram of an implantable device incorporating anembodiment of the invention.

FIG. 6 schematically illustrates a reader and an inductively linkedtelemetry system in accordance with embodiments of the invention.

FIG. 7 is a workflow diagram illustrating a procedure for establishingthe resonance frequency of a telemetry receiver in accordance withembodiments of the invention.

FIG. 8 is a workflow diagram illustrating a procedure for establishingthe resonance frequency of a telemetry receiver for the first time.

FIG. 9 schematically illustrates a functional flow chart of aphase-locked loop configuration in accordance with embodiments of theinvention.

DETAILED DESCRIPTION

A representative environment for a telemetry application in accordanceherewith is an implantable drug-delivery pump as illustrated in FIG. 5.The drug pump device 500 includes a cannula 502 and a pair of chambers504, 506. The top chamber 504 defines a drug reservoir that contains thedrug to be administered in liquid form, and the bottom chamber 506contains a liquid which, when subjected to electrolysis usingelectrolysis electrodes, evolves a gaseous product. The two chambers areseparated by a corrugated diaphragm (not shown). The cannula 502connects the top drug chamber 504 with a check valve 514 inserted at thesite of administration. Control circuitry 518, a battery 520, and atelemetry system 522 for power and data transmission are embedded withinthe device 500. Depending on the complexity of the control functionalityit provides, the control circuitry 518 may be implemented, e.g., in theform of analog circuits, digital integrated circuits (such as, e.g.,microcontrollers), or programmable logic devices, and the telemetrysystem 522 may be integrated in whole or in part within the controlcircuitry 518. In some embodiments, the control circuitry 518 includes amicroprocessor and associated memory for implementing complexdrug-delivery protocols. The drug pump device 500 may also includevarious sensors (e.g., pressure and flow sensors) for monitoring thestatus and operation of the various device components, and such data maybe logged in the memory for subsequent retrieval and review. For theprolonged use of the drug pump device 500 following implantation, thedevice includes one or more fill ports 524 in fluid communication withthe drug reservoir 504, which permit a refill needle (not shown) to beinserted therethrough. Additional features of implantable drug-deliverypumps usable in accordance with embodiments of the present invention maybe found in U.S. Pat. No. 8,231,608, filed on May 8, 2009, the entiredisclosure of which is incorporated by reference herein.

With reference to FIG. 6, a representative telemetry system (or“receiver”) 600 communicates inductively with a reader (or“transmitter”) 605 over a narrowband link. The reader 605 includes atuning circuit (which includes or consists essentially of, for example,a capacitance (e.g., one or more capacitors) C_(Tuning) and aninductance (e.g., one or more inductors) L_(Reader)), an AC signalsource (or “oscillator”) 610, and support (or “control”) circuitry 614that may include an optional memory that may include volatile and/ornon-volatile components. The internal resistance (and additionalimpedance) of the reader 605 is represented as a resistance R_(Reader).The inductor L_(Reader) may serve as the system antenna, or the reader605 may utilize a separate antenna. The telemetry system 600 includes acapacitance (e.g., one or more capacitors) C_(Telemetry), an inductance(e.g., one or more inductors) L_(Telemetry), and support circuitry 624that may include or consist essentially of, e.g., a microcontroller ormicroprocessor and a computer memory having volatile and nonvolatilecomponents. The internal resistance (and additional impedance) of thetelemetry system 600 is represented as a resistance R_(Telemetry). Asnoted earlier, these components may be discrete or may be within thecontrol circuitry 518 described above. In operation, C_(Tuning) andL_(Reader) form a resonator (or “resonator circuit”) driven by the ACsource 610. When inductively coupled to the telemetry system 600 via theinductor L_(Telemetry), the signal produced by the AC source suppliespower and/or a data signal to the telemetry system 600, charging thebattery 520 and operating or communicating with the control circuitry518 (see FIG. 5). Because the inductive link is narrowband, it isimportant for resonance frequencies of the systems 600, 605 to match asclosely as practicable, and for the AC source 610 to operate atsubstantially this frequency. Data signals may be transmitted to thetelemetry system 600 by amplitude modulation of the time-varying ACsignal (where the AC frequency is fixed and data is encoded in anamplitude-modulated signal envelope) or frequency modulation of thesignal (where the AC frequency is varied in accordance with thetransmitted data, but does not deviate significantly from the resonancefrequency).

In practice, both L and C components have manufacturing tolerances thattypically preclude a precise match of the resonance frequencies. Thisdifference or shift may be tolerable as long as the effective resonancefrequency remains within the frequency band allowed by regulation (i.e.,119 kHz to 135 kHz or other allowable medical frequency band). Selectinga tuning capacitor with a rated ±1% tolerance provides a tolerancebudget for the coil inductance (which is more difficult to control) ofup to ±10%. Typically, however, tighter control of the resonancefrequency is necessary or desirable to avoid excessive attenuation in anarrowband link. In one approach, the support circuitry 624 contains aplurality of capacitors that may each be selectively activated ordeactivated. For example, the capacitors may be arranged in parallelwith fusible links. During system testing following manufacture,selected ones of the capacitors are deactivated by, for example, laserexposure or localized application of a voltage pulse, and the finalcapacitance of the sum of the capacitances of the capacitors that havenot been deactivated. Alternatively, each capacitor may be connected atransistor (e.g., a MOSFET) configured as a switch, and which isoperated by the support circuitry 624 (e.g., by a microcontrollertherewithin). During final testing, the proper capacitance (i.e., thecapacitance at which the correct resonance frequency is attained) isdetermined and a binary “image” of the switch states corresponding thiscapacitance is permanently stored in the memory of the support circuitry624 as the pedigree of the device; when the device is powered on, thisvalue is loaded and the transistors are operated accordingly.

This approach may not be practical for a very small device because ofthe space required for multiple capacitors and their switches. For sucha device, an advantageous approach is to minimize the number ofcomponents in the device itself, design their tolerances such that theresonance frequency is always within the allowed band and push thefrequency-tracking complexity to the reader 605, which has fewer sizeconstraints. Again, because of the narrowband link, the bandwidth istypically limited to only a few kHz to maximize the Q factor and therebypermit sufficient power transfer, and the resonance frequencies of thereader 605 and the telemetry system 600 should be accurately matched.

Thus, in an alternative embodiment, the multiple capacitors are disposedwithin the support circuitry 614 of the transmitter 605 rather than inthe receiver 600. To keep the receiver small, no adjustment is made toits resonance frequency; the component tolerances are specified suchthat the resonance frequency will always fall within an acceptablefrequency range. The transmitter 605 is tuned—e.g., the totalcapacitance is varied—until a matching frequency is achieved. In thisway, a single reader 605 can be used with many receivers 600, and canstore, in non-volatile memory of the support circuitry 614, a table ofreceiver identifiers each associated with the determined capacitance (orresonance frequency) for that receiver. When a new receiver sessionbegins, the reader 605 interrogates the receiver 600, obtains itsidentifier, and if it locates the identifier in its stored look-uptable, it activates the proper number of capacitors to achieve thestored value. If the identifier is not found, the transmitter 605 istuned.

An embodiment of a tuning process, performed to discover the resonancefrequency of the receiver and store its value for future use, isillustrated in FIG. 7. Some or all of the steps may occur during themanufacturing process (e.g., prior to implant of the device within apatient). In a first step 710, the transmitter initiates communicationwith the receiver at an expected resonance frequency. This frequency maynot be optimal in terms of power transfer, but will typically besufficiently close to permit communication under static conditions. Ifthe receiver does not respond, however, the transmitter frequency isaltered (step 715) by the support circuitry 614 (see FIG. 6), and steps710 and 715 repeat until communication is established. At this point(step 720), the transmitter interrogates the receiver to determinewhether the latter can transmit a resonance frequency that has beendetermined and/or set during manufacture. If so (step 725), thetransmitter receives the frequency and sets its internal oscillationfrequency accordingly (step 730). Alternatively or in addition, thereceiver may obtain an identifier from the receiver. If the transmitterreceives both an identifier and a resonance frequency, it may save thetuple as a database record in nonvolatile storage. If the receiverprovides no resonance frequency, it may still provide an identifier,such as a serial number. For medical devices, this is documented as partof the device master record and subsequently transferred to the patientrecord in the case of an implant. This allows for easy access in thefuture from the cloud or other server system.

It should be understood that wireless communication and reporting ofdata (such as the resonance frequency) can occur actively or passively.In an active system, communication is bidirectional and the receivertransmits data to the transmitter over the wireless link. In a passivesystem, the receiver modulates the received signal in accordance withthe data to be transferred; this modulation is detected and interpretedby the transmitter. For example, the receiver may modulate the powertransfer by selectively switching, in a pattern corresponding to data, aresistance into the LC resonator.

If the receiver has not stored its resonance frequency, the procedureillustrated in FIG. 8 may be undertaken. In steps 810, 815, thetransmitter varies its oscillation frequency until communication withthe receiver is established. To facilitate tuning, the transmitter andreceiver establish synchronization. This begins with a “tune start”command sent by the transmitter (step 820), which is received (step 825)and causes the receiver to begin recording the signal strengths ofcommunications received from the transmitter (step 830). The transmittersends signals that sweep through a band of frequencies around theexpected resonance frequency (step 835); for example, the frequency bandmay reflect the maximum expected frequency variation given manufacturingtolerance limits. The amplitudes of these signals are recorded by thereceiver. In some embodiments, the receiver is configured to detect thepeak signal strength (step 840) and transmit the corresponding frequencyback to the transmitter (step 845) when interrogated for the result bythe transmitter (step 850). In other embodiments, the receiver simplyrecords a log of signal strengths (and, in some embodiments,corresponding sensed frequencies), and sends the log back in response tothe interrogation signal; in this case, the transmitter is programmed toexamine the log and identify the frequency with the peak signal strength(i.e., the largest log entry). The transmitter selects this frequency asits oscillation (transmission) frequency (steps 855, 860) and uses thisfrequency during its communication session with the receiver. In someembodiments, the transmitter also sends data identifying this frequencyto the receiver (step 865), which records it (step 870) for subsequenttransmission upon interrogation (FIG. 7, step 725) so that the procedureof FIG. 7 can subsequently be used. Alternatively or in addition, thetransmitter may associate the retrieved serial number with thediscovered resonance frequency in a database record.

In addition to signal strengths (i.e., amplitudes), log entries maystore frequency values, as suggested above, or may instead store indicesthat correspond to those values. This approach limits the amount datastored on receiver. For example, the indices may be simple ascendingintegers or may otherwise correspond to elapsed times during thefrequency sweep (i.e., the log may be time-indexed). Alternatively, theindices may be hashed values based on the time, the frequency and/orother relevant parameters, and which the transmitter may use toreconstruct or retrieve the parameters. In this way, the transmitter mayaccess multiple relevant parameters without necessitating their storageon the receiver.

In various embodiments of the invention, the transmitter determines theresonance frequency without receiving a pre-set, stored resonancefrequency or the above-described log from the receiver. In suchembodiments, the transmitter sends signals that sweep through a band offrequencies around the expected resonance frequency, as described forstep 835 above. During the frequency sweep, signal strengths are notnecessarily detected or stored by the receiver. Instead, the transmitterdetermines the resonance frequency by detecting the point during thefrequency sweep when the transmitter loading (i.e., the powertransferred by the transmitter over the resonant link with the receiver)is maximized. The maximum loading may be detected and the correspondingfrequency identified by any suitable method, e.g., repeated comparisonsduring the sweep of a current loading level to a previously determinedmaximum.

FIG. 9 illustrates the components and functional flow of a circuit 900using a conventional phase-locked loop (PLL) 905 in the transmitterdesign to maintain the oscillator at the resonant frequency. Here, thecarrier frequency of the transmitter is not set by the oscillator 910 asin a traditional implementation, but instead the oscillator 910 tracksthe resonance frequency of the transmitter 605. The circuit 900, whichmay be included within the support circuitry 614 of the transmitter (seeFIG. 6), includes a control circuit block 915, a resonator circuit block920, an amplifier 925, and an amplitude detector 930. As shown, thecontrol circuit block 915 may include the PLL 905, the oscillator 910,and a low-pass filter 935. The illustrated loop configuration maintainsa target frequency 940, i.e., the transmitter's resonance frequency,obtained from the receiver during setup or determined as describedabove. A tuning circuit (or “tuning block”) 945 in the resonator circuitblock 920 may operate in the manner described above, selectivelyswitching in capacitors arranged in parallel to achieve and maintain thetarget frequency; or by adjusting the inductance—e.g., by selectivelyadvancing a magnetically permeable core into the interior of a coil 950.The further the core is advanced into the coil (e.g., by a steppermotor), the greater will be the inductance. An advantage to adjustingthe inductance in this way is that the adjustment may be continuous oressentially so, as contrasted with the step adjustment to capacitance.The PLL adjusts the oscillator frequency such that the amplitude ofoscillation at the coil is maximized, i.e., matching the resonancefrequency. The detector 930 measures the amplitude of oscillation by,for example, extracting it using an envelope detector followed byfiltering by the filter 935 to remove any noise and modulation.

It should be emphasized that the control and support circuitry describedabove may be implemented in hardware, software or a combination of thetwo. For embodiments in which the functions are provided as one or moresoftware programs, the programs may be written in any of a number ofhigh level languages such as FORTRAN, PYTHON, JAVA, C, C++, C#, BASIC,various scripting languages, and/or HTML. Additionally, the software canbe implemented in an assembly language directed to the microprocessor;for example, the software may be implemented in Intel 80×86 assemblylanguage if it is configured to run on an IBM PC or PC clone. Thesoftware may be embodied on an article of manufacture including, but notlimited to, a floppy disk, a jump drive, a hard disk, an optical disk, amagnetic tape, a PROM, an EPROM, EEPROM, field-programmable gate array,or CD-ROM. Embodiments using hardware circuitry may be implementedusing, for example, one or more FPGA, CPLD or ASIC processors.

Various embodiments of the invention are described above. It will,however, be apparent to those of ordinary skill in the art that otherembodiments incorporating the concepts disclosed herein may be usedwithout departing from the spirit and scope of the invention.Accordingly, the above description is intended to be only illustrativeand not restrictive.

What is claimed is:
 1. An implantable device comprising: an electronicactuator; a cannula for conducting fluid from the electronic actuator toan implant site; a battery for powering the electronic actuator; controlcircuitry for controlling operation of the electronic actuator; and atelemetry system, operatively coupled to the battery and to the controlcircuitry, for (i) wirelessly receiving commands for controllingoperation of the electronic actuator over a narrowband inductive linkand (ii) wirelessly receiving a power signal for charging the batteryover the narrowband inductive link, the telemetry system comprising amemory for storing a resonance frequency of the system and telemetrycontrol circuitry for wirelessly reporting the stored resonancefrequency in response to an external interrogation signal.
 2. The deviceof claim 1, wherein the telemetry control circuitry is furtherconfigured to detect a synchronization signal and, in response thereto,to detect (i) a succession of transmitted frequencies and (ii)associated amplitudes thereof, and to store data indicative of thedetected frequencies in a log in the memory.
 3. The device of claim 2,wherein the telemetry control circuitry is further configured totransmit the stored log over the narrowband link in response to awirelessly transmitted request therefor.
 4. The device of claim 3,wherein the telemetry control circuitry is further configured to receivethe resonance frequency of the system over the narrowband link followingtransmission of the stored log, and to store the received resonancefrequency in the memory.
 5. The device of claim 1, wherein the telemetrycontrol circuitry is further configured to detect a synchronizationsignal and, in response thereto, to (i) detect a succession oftransmitted frequencies, (ii) determine an associated amplitude of eachdetected frequency, (iii) determine a largest one of the amplitudes, and(iv) store the frequency associated with the largest amplitude in thememory as the system resonance frequency.
 6. The device of claim 1,wherein the electronic actuator comprises a pump.
 7. A reader forwirelessly communicating with an implanted device over a narrowbandinductive link, the reader comprising: a resonator circuit; and controlcircuitry for operating the resonator circuit to (i) communicate withthe implanted device over a narrowband link, (ii) determine a resonancefrequency of the implanted device, and (iii) charge the implanted deviceover the narrowband link at the determined resonance frequency.
 8. Thereader of claim 7, wherein the control circuitry is configured todetermine the resonance frequency of the implanted device by wirelessinteraction therewith, and is further configured to store the determinedresonance frequency in a database in association with an identifier ofthe implanted device.
 9. The reader of claim 7, wherein the controlcircuitry is configured to determine the resonance frequency of theimplanted device by (i) transmitting a wireless signal whose frequencyvaries over time and (ii) detecting, during the transmission, a peakamplitude of the wireless signal whose frequency corresponds to theresonance frequency.
 10. The reader of claim 7, wherein the controlcircuitry is configured to determine the resonance frequency of theimplanted device by (i) transmitting a wireless signal whose frequencyvaries over time, (ii) receiving, from the implanted device, a log ofentries each having an amplitude corresponding to a frequency detectedduring the transmission, and (iii) determining which of the entriescorresponds to the largest amplitude.
 11. The reader of claim 10,wherein the log is time-indexed, and the resonance frequency isdetermined by matching the log entry corresponding to the largestamplitude with a corresponding frequency of the wireless signal.
 12. Thereader of claim 7, further comprising phase-locked loop circuitry formaintaining transmission at the resonance frequency.
 13. The reader ofclaim 12, wherein the phase-locked loop circuitry comprises anamplifier, a tuning circuit, an amplitude detector, and a filter. 14.The reader of claim 13, wherein the tuning circuit varies at least oneof a capacitance or an inductance of the resonator circuit.
 15. Thereader of claim 7, wherein the resonator circuit comprises anoscillator, a capacitance, and an inductance.
 16. A method ofintercommunication over a narrowband inductive link between an externalreader and an implanted device, the method comprising the steps of:determining, by the reader over the inductive link, a resonancefrequency of the implanted device; and communicating with and chargingthe implanted device over the narrowband link at the determinedresonance frequency.
 17. The method of claim 16, wherein the determiningstep comprises transmitting, by the reader, a wireless signal whosefrequency varies over time and detecting, during the transmission, apeak amplitude of the wireless signal whose frequency corresponds to theresonance frequency.
 18. The method of claim 16, wherein the determiningstep comprises: transmitting, by the reader, a wireless signal whosefrequency varies over time; recording, by the implanted device, a log ofentries each having an amplitude corresponding to a frequency detectedduring the transmission; receiving, by the reader, at least a portion ofthe log from the implanted device over the narrowband link; anddetermining, by the reader, which of the entries corresponds to thelargest amplitude.
 19. The method of claim 18, wherein the log istime-indexed, and determining the resonance frequency of the implanteddevice comprises matching the entry corresponding to the largestamplitude with the corresponding frequency of the wireless signal. 20.The method of claim 16, further comprising the step of maintainingtransmission at the resonance frequency using a phase-locked loop. 21.The method of claim 16, further comprising the step of storing, by thereader, the determined resonance frequency in a database in associationwith an identifier of the implanted device.